Read the passage carefully and answer the questions based on the passage:
The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction. In the rate equation Rate = \( k [A]^x [B]^y \)
\( x \) and \( y \) indicate how sensitive the rate is to the change in concentration of A and B, respectively. Sum of these exponents, i.e., \( x + y \) gives the overall order of a reaction where \( x \) and \( y \) represent the order with respect to the reactants A and B, respectively. Hence, the sum of powers of the concentration of the reactants in the rate law expression is called the order of that chemical reaction. For a first order reaction, the concentration of the reactant varies as \( [R] = [R]_0e^{-kt} \)
Which of the following is a zero order reaction?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Decomposition of gaseous ammonia on a hot platinum surface
In a zero order reaction the rate does not depend on the concentration of the reactant: Rate = \( k[\mathrm{R}]^{0} = k \). Such reactions are uncommon; they occur mainly on metal surfaces and in some enzyme-catalysed reactions, where the surface (or the enzyme) is the limiting factor.
At high pressure, ammonia covers the whole platinum surface. Adding more ammonia cannot raise the amount adsorbed, so the rate stays constant:
\( \displaystyle \ce{2NH3(g)} \xrightarrow[\text{1130 K}]{\text{Pt catalyst}} \ce{N2(g) + 3H2(g)} \)
\( \displaystyle \text{Rate} = k[\ce{NH3}]^{0} = k \)
So the decomposition of gaseous ammonia on a hot platinum surface is the zero order reaction.
Why the other options are incorrect
Option (1) Radioactive decay of unstable nuclei — Incorrect. All radioactive decay follows first order kinetics; the rate is proportional to the number of nuclei present.
Option (2) Hydrogenation of ethene — Incorrect. \( \ce{C2H4 + H2 -> C2H6} \) has Rate = \( k[\ce{C2H4}] \), a first order reaction.
Option (4) Decomposition of \(N_2O_5\) — Incorrect. \( \ce{2N2O5 -> 4NO2 + O2} \) is first order in \( \ce{N2O5} \), as Q7 of this paper also shows.
NCERT (2026-27 edition):
Zero Order Reactions – …
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | 15-05 S2 | – | 14-06 S3 | – |